In general, the combustion types of internal combustion engines are categorized into two types, i.e., the premixed combustion type and the diffusion combustion type. The premixed combustion type is the one in which as exemplified by a gasoline engine, a fuel-air mixture obtained by preliminarily mixing fuel and air is combusted by spark ignition. In contrast, the diffusion combustion type is the one in which as exemplified by a diesel engine, fuel is supplied to high-temperature and high-pressure compressed air so that the fuel is combusted while the fuel and the air are diffused and mixed with each other. Furthermore, in recent years, the premixed compression-ignition combustion type has been drawing attention, as a new combustion type, and development of putting the premixed compression-ignition combustion into practice has been promoted. The premixed compression-ignition combustion type is the one in which a fuel-air mixture obtained by preliminarily mixing fuel and air is combusted by not spark ignition but self-ignition through compression; the premixed compression-ignition combustion is referred to as the HCCI combustion (Homogeneous-Charge Compression Ignition Combustion) and is expected as a next-generation combustion type.
In each of the various kinds of combustion types, application of active species such as an ozone gas and the like to promotion of combustion in an internal combustion engine has been studied. It is allegedly claimed that use of the strong oxidation effect of ozone makes it possible to realize the rise of ignitability in a cylinder of the internal combustion engine, the promotion of combustion, and improvement of gasoline mileage of the internal combustion engine. For example, PLT 1 listed below disclosed an internal combustion engine in which part of intake air is ozonized through silent discharge by use of an ozone generator provided in an intake pipe (intake manifold) of an internal combustion engine and then the intake air including the ozone is supplied into the cylinder. In the ozone generator disclosed in PLT 1, a discharge space through which intake air flows is provided between a ground electrode and a high-voltage electrode that are arranged in such a way as to face each other; silent (space) discharge produced in the discharge space ozonizes the intake air.
As is the case with PLT 1, PLT 2 listed below discloses an internal combustion engine in which an ozone generator is provided in the air inlet of an air cleaner. The ozone generator disclosed in PLT 2 includes a linear discharge electrode provided on the surface of an inductor and an electrode plate provided with induction electrodes that are arranged in such a way as to face each other while pinching the inductor. Creeping discharge produced, around the discharge electrode ozonizes part of intake air, and then the intake air including the ozone is supplied into a cylinder.